Often, in a system with such a rotating joint, it is the case that two separate waveguide propagation lines carry two separate microwave signals, and it is a requirement that the two separate microwave signals be passed through the rotary joint, simultaneously. It is desirable under such a requirement that the rotary joint would: first, accept the two microwave signals at two separate waveguide inlet ports; second, propagate the two signals simultaneously in a common waveguide through the rotary joint; and third, deliver the two signals at two separate waveguide outlet ports.
In some cases, the two microwave signals may pass through the rotary joint in opposite directions; for example, transmit and receive signals. In other cases, the two signals may pass both in the same direction through the rotary joint. In both cases, it is important that there should be substantially no spurious propagation modes generated by the passage of either of the two signals, at or by the joint, of such a nature as could lead to undue attenuation of either signal, or as could interfere with, or be detected as a component of, the other signal.